In order to improve thermal efficiency of gasoline internal combustion engines, lean burn is known to give enhanced thermal efficiency by reducing pumping losses and increasing the ratio of specific heat. Generally speaking, lean burn is known to give low fuel consumption and low NOx emissions. There is however a limit at which an engine can be operated with a lean air/fuel mixture because of misfire and combustion instability as a result of a slow burn. Known methods to extend the lean limit include improving ignitability of the mixture by enhancing the fuel preparation, for example using atomised fuel or vaporised fuel, and increasing the flame speed by introducing charge motion and turbulence in the air/fuel mixture. Finally, combustion by auto-ignition, or homogeneous charge compression ignition, has been proposed for operating an engine with very lean or diluted air/fuel mixtures.
When certain conditions are met within a homogeneous or close to homogenous charge of lean air/fuel mixture during low load operation, homogeneous charge compression ignition can occur wherein bulk combustion takes place initiated simultaneously from many ignition sites within the charge, resulting in very stable power output, very clean combustion and high fuel conversion efficiency. NOx emission produced in controlled homogeneous charge compression ignition combustion is extremely low in comparison with spark ignition combustion based on propagating flame front and heterogeneous charge compression ignition combustion based on an attached diffusion flame. In the latter two cases represented by spark ignition engine and diesel engine, respectively, the burnt gas temperature is highly heterogeneous within the charge with very high local temperature values creating high NOx emission. By contrast, in controlled homogeneous charge compression ignition combustion where the combustion is uniformly distributed throughout the charge from many ignition sites, the burnt gas temperature is substantially homogeneous with much lower local temperature values resulting in very low NOx emission.
Engines operating under controlled homogeneous charge compression ignition combustion have already been successfully demonstrated in two-stroke gasoline engines using a conventional compression ratio. It is believed that the high proportion of burnt gases remaining from the previous cycle, i.e., the residual content, within the two-stroke engine combustion chamber is responsible for providing the hot charge temperature and active fuel radicals necessary to promote homogeneous charge compression ignition in a very lean air/fuel mixture. In four-stroke engines, because the residual content is low, homogeneous charge compression ignition is more difficult to achieve, but can be induced by heating the intake air to a high temperature or by significantly increasing the compression ratio. This effect can also be achieved by retaining a part of the hot exhaust gas, or residuals, by controlling the timing of the intake and exhaust valves.
Homogeneous charge compression ignition combustion of a gasoline like fuel (or petrol like, or fuel with high octane number) requires a temperature of approximately 1100 K to achieve auto ignition. While it may be possible to operate in the HCCI mode over a significantly wide operating range of engine speeds and load, starting in the HCCI mode is considerably more difficult due to cold engine parts causing large heat transfer losses from the charge, and the absence of heat sources. Using the above solution, relying on exhaust gas from the previous combustion, a mode switch to normal spark ignition combustion will have to take place following every fuel cut-off, since no exhaust gas from a previous combustion will be available once the engine is to be restarted.
U.S. Pat. No. 6,336,436 discloses a gasoline engine having an actuating system including an in-cylinder fuel injection system and an ignition system, capable of changing over combustion between spark ignition combustion and compression auto ignition combustion, and a controlling system for controlling the combustion changeover. In a transition from one combustion mode to the other, the actuating system is controlled to perform transient combustion such as stratified charge combustion with fuel injection on the compression stroke, or combustion with fuel injection during a valve shutoff period during which intake and exhaust valves are both closed.
Although U.S. Pat. No. 6,336,436 deals with the transition between normal spark ignition combustion and compression auto ignition combustion and vice versa, it relies on stratified combustion as an intermediate mode to be able to move actuators, such as the throttle, to a desired setting without causing misfire. However, U.S. Pat. No. 6,336,436 is silent regarding the transition from no combustion to compression auto ignition combustion.